Beta-Ga2O3 film injected with hydrogen ions as well as preparation method and application of beta-Ga2O3 film

Abstract

The invention discloses a beta-Ga2O3 film injected with hydrogen ions and a preparation method and application thereof, and belongs to the technical field of semiconductors. According to the preparation method, the oxygen vacancy defect in the MOCVD technology-based homoepitaxial non-doped gallium oxide beta-Ga2O3 is filled by adopting monovalent hydrogen cation (H < + >) injection, so that the electron concentration and the electron mobility are improved. According to the invention, monovalent hydrogen cations (H < + >) are correspondingly injected into a substrate with n-type electrical property, gap-type H impurities (Hi) and substitutive H impurities (Ho) are formed by injecting hydrogen ions, the substitutive H impurities (Ho) replace O positions on beta-Ga2O3 crystal lattices, and a part of substitutive H impurities (Ho) compensate for oxygen vacancy defects in gallium oxide. Gap H impurities and substitutive H impurities for ion implantation play a role of shallow energy level donors, so that the electron carrier concentration is improved, and the electron mobility is effectively improved to 1.9 times of the original value.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductor preparation, and in particular relates to a hydrogen ion-injected beta-Ga2O3 film and a preparation method and application thereof. Background Art

[0002] Gallium oxide (Ga2O3) has an ultra-wide band gap of ~4.9 eV. In recent years, it has become a research hotspot in academia and industry due to its excellent application potential in high-voltage and high-power devices (field-effect transistors, Schottky barrier diodes), optoelectronics (phosphors and electroluminescent devices, solar-blind photodetectors), memories (spintronics devices, resistive random access memory devices), and sensing systems (gas sensors, radiation detectors).

[0003] Based on homoepitaxial technology, that is, the non-doped β-Ga2O3 gallium oxide film is grown on a gallium oxide substrate by metal organic chemical vapor deposition (MOCVD). Although it is not doped intentionally, due to the high temperature during the MOCVD epitaxial growth process, the oxygen in β-Ga2O3 escapes from the lattice position and forms oxygen vacancies (Vo) in the β-Ga2O3 lattice. Oxygen vacancies (Vo) play a donor role in β-Ga2O3. Under the action of oxygen vacancies, gallium oxide exhibits n-type electrical properties. However, the donor energy level formed by oxygen vacancies (Vo) is in a deeper region of the forbidden band in the gallium oxide energy band, far from the bottom of the conduction band. It is a deep donor energy level with a large donor ionization energy. Therefore, the concentration of electron carriers formed is not very high, only 10 14 ~ 10 15 cm -3 If β-Ga2O3 is to be used as the body region of a high-voltage and high-power device, the order of magnitude of the lower carrier concentration formed by the action of oxygen vacancies (Vo) is difficult to meet the requirements. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the above-mentioned prior art and to provide a β-Ga2O3 film injected with hydrogen ions and a preparation method and application thereof, so as to solve the problem in the prior art that when β-Ga2O3 is used as the body region of a high-voltage and high-power device, the lower carrier concentration formed only by the action of oxygen vacancies (Vo) is difficult to meet the electron carrier demand.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: A method for preparing a β-Ga2O3 film implanted with hydrogen ions comprises the following steps: Step 1, growing a non-doped β-Ga2O3 gallium oxide thin film on a Ga2O3 substrate; Step 2, pretreating the non-doped β-Ga2O3 gallium oxide film; Step 3, injecting monovalent hydrogen ions into the pretreated non-doped β-Ga2O3 gallium oxide film to form a β-Ga2O3 film injected with hydrogen ions, wherein the concentration of monovalent hydrogen ions in the β-Ga2O3 film injected with hydrogen ions is 1-8ppm.

[0006] A further improvement of the present invention is: Preferably, in step 3, the injection temperature is room temperature.

[0007] Preferably, in step 3, the implantation energy is 90 KeV.

[0008] Preferably, in step 3, the implantation depth is 400 nm.

[0009] Preferably, in step 3, the injection inclination angle is 0°.

[0010] Preferably, in step 1, the thickness of the epitaxial layer in the doped β-Ga2O3 gallium oxide film is 400 nm.

[0011] Preferably, in step 1, the Ga2O3 substrate is grown by metal organic chemical vapor deposition to obtain a non-doped β-Ga2O3 gallium oxide thin film.

[0012] Preferably, in step 2, the pretreatment process is ultrasonic cleaning using acetone, ethanol and deionized water in sequence.

[0013] A β-Ga2O3 film implanted with hydrogen ions, prepared by any one of the preparation methods described above, wherein the β-Ga2O3 gallium oxide film is doped with hydrogen ions.

[0014] An application of the above-mentioned β-Ga2O3 film implanted with hydrogen ions, wherein the β-Ga2O3 film implanted with hydrogen ions is used to prepare the body region of a high-voltage and high-power device, or the body region of a photodiode.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention discloses a method for preparing a β-Ga2O3 film implanted with hydrogen ions. The method adopts monovalent hydrogen cations (H + ) is injected to fill the oxygen vacancy defects in the homoepitaxial non-doped gallium oxide β-Ga2O3 based on MOCVD technology, thereby increasing the electron concentration and electron mobility. The present invention will present the corresponding monovalent hydrogen cation (H +) is injected. By injecting hydrogen ions, interstitial H impurities (Hi) and substitutional H impurities (Ho) are formed. The substitutional H impurities (Ho) replace the position of O on the β-Ga2O3 lattice, and a part of the substitutional H impurities (Ho) compensates for the oxygen vacancy defects in gallium oxide. The interstitial H impurities and substitutional H impurities injected by ions both act as shallow energy level donors, which not only increase the electron carrier concentration, but also effectively increase the electron mobility to 1.9 times the original value. After ion injection, the Hall test was performed again, and the Hall electron carrier concentration increased from 1.2 × 10 15 cm -3 Increased to 9.04 × 10 16 cm -3 , and the electron mobility increased from 63 cm 2 / V·s increased to 119 cm after ion implantation 2 / V·s. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A cross-sectional view of the crystal structure showing oxygen vacancies in gallium oxide before hydrogen ion implantation; Figure 2 H + Schematic diagram of the cross section of the β-Ga2O3 film obtained by ion implantation and non-doped MOCVD epitaxy and the cross section of the ohmic electrode; Figure 3 is a non-doped single-crystalline β-Ga2O3 thin film obtained based on MOCVD. + Comparison of X-ray diffraction (XRD) 2θ patterns before and after ion implantation; Figure 4 is a non-doped single-crystalline β-Ga2O3 thin film obtained based on MOCVD. + Comparison of X-ray diffraction (XRD) rocking curve patterns on the (400) crystal plane before and after ion implantation; Figure 5 H is the value under vacuum temperature test conditions. + The Hall electron carrier concentration of β-Ga2O3 film before and after ion implantation varies with the test temperature; Figure 6 H is the value under vacuum temperature test conditions. + The Hall electron mobility of β-Ga2O3 film before and after ion implantation varies with the test temperature; Figure 7 H is the value under vacuum temperature test conditions. + The resistivity of β-Ga2O3 film before and after ion implantation varies with the test temperature; Figure 8 H is the value under vacuum temperature test conditions. +Donor ionization energy of β-Ga2O3 film before ion implantation; Fig. 9 H is the value under vacuum temperature test conditions. + Donor ionization energy of β-Ga2O3 film after ion implantation. DETAILED DESCRIPTION

[0017] The present invention is further described in detail below in conjunction with the accompanying drawings: In order to enable those skilled in the art to understand the characteristics and effects of the present invention, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used in the text are the common meanings understood by those skilled in the art for the present invention. In the event of a conflict, the definition in this specification shall prevail.

[0018] In this document, unless otherwise specified, “includes,” “including,” “contains,” “has,” or similar terms cover the meanings of “consisting of” and “mainly consisting of,” for example, “A includes a” covers the meanings of “A includes a and other” and “A only includes a.”

[0019] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0020] The following examples use conventional instruments and equipment in the art. The experimental methods in the following examples where specific conditions are not specified are usually carried out under conventional conditions or under conditions recommended by the manufacturer. The various raw materials used in the following examples are conventional commercial products unless otherwise specified, and their specifications are conventional specifications in the art. In the specification of the present invention and the following examples, unless otherwise specified, "%" means weight percentage, "part" means weight part, and ratio means weight ratio.

[0021] As the question raised in the background technology, first of all, MOCVD, as an advanced vapor phase epitaxial growth technology, can grow high-quality, high-purity crystalline films on substrates by precisely controlling parameters such as the flow rate, pressure and temperature of the reaction gas. In the homoepitaxial growth process of gallium oxide, although no doping treatment is specifically performed, due to the high temperature environment required for MOCVD epitaxial growth (usually up to several hundred to thousands of degrees Celsius), this extreme condition causes some oxygen atoms in the β-Ga2O3 crystal to obtain sufficient energy, thereby escaping from the original lattice position and overflowing, and finally leaving oxygen vacancies (Vo) in the βGa2O3 lattice. These oxygen vacancies (Vo) play the role of donors in the β-Ga2O3 crystal, that is, they can provide additional electrons, making the gallium oxide film exhibit n-type electrical properties. This is because in semiconductor materials, donor impurities or defects can donate electrons to the conduction band, thereby increasing the electron concentration in the conduction band and making the material exhibit n-type conductivity. However, it is worth noting that the donor energy level formed by oxygen vacancies (Vo) is located in a deeper region of the band gap in the energy band structure of gallium oxide, far away from the bottom of the conduction band. This deep donor energy level means that the donor ionization energy is large, that is, a higher energy is required to excite the electrons on the donor energy level into the conduction band to form free electrons. Therefore, although oxygen vacancies can provide electrons, the concentration of the formed electron carriers is not very high, usually only about 10 14 ~10 15 cm -3 This level of carrier concentration may be sufficient for some applications, but it cannot meet the needs of high-voltage and high-power devices that require high carrier concentrations.

[0022] In order to solve the above problems, the first aspect of the present invention discloses a method for preparing a β-Ga2O3 film injected with hydrogen ions, which is a method for effectively filling oxygen vacancy defects in gallium oxide by hydrogen cation injection, and the method comprises the following steps: Step 1, growing a non-doped β-Ga2O3 gallium oxide film on a Ga2O3 substrate; in the present invention, a non-doped gallium oxide (β-Ga2O3) film is used, which is obtained by homoepitaxial growth based on MOCVD technology. The film itself exhibits n-type electrical properties due to oxygen vacancy (Vo) defects. The thickness of the epitaxial layer is 400 nm, and its Hall electron carrier concentration is 1.2×1015 cm at room temperature. -3 , the electron mobility is 63 cm2 / V·s.

[0023] Step 2, in the embodiment of the present invention, the non-doped homoepitaxial gallium oxide (β-Ga2O3) film is firstly ultrasonically cleaned using a process of acetone (10 minutes) → ethanol (10 minutes) → deionized water (10 minutes) to remove impurities on the surface of the film.

[0024] Step 3, injecting monovalent hydrogen ions into the pretreated non-doped β-Ga2O3 gallium oxide film to form a β-Ga2O3 film injected with hydrogen ions, wherein the concentration of monovalent hydrogen ions in the β-Ga2O3 film injected with hydrogen ions is 1-8ppm.

[0025] The specific process is to put the cleaned sample into the ion implanter ULVAC IMX-3500 and implant it with monovalent hydrogen cations (H + ) as the ion source, and ion implantation was performed perpendicular to the sample surface (i.e., the incident angle Tilt = 0°). The parameters of ion implantation are shown in Table 1: Table 1 H + Ion implantation parameter table

[0026] In the present invention, the implantation depth is consistent with the thickness of the epitaxial layer, so that the entire epitaxial layer has hydrogen ions.

[0027] The present invention is carried out + After the ion implantation, the rapid thermal annealing for the purpose of impurity activation after the ion implantation is intentionally not performed. Usually, after the ion implantation, the rapid thermal annealing for the purpose of impurity activation is performed at a temperature range of 600 to 900°C. However, this higher temperature range required for activating impurities may generate new oxygen vacancy defects again. In order to avoid generating new oxygen vacancy defects, the present invention intentionally does not perform rapid thermal annealing for activating impurities after the ion implantation.

[0028] The second aspect of the present invention discloses a β-Ga2O3 film implanted with hydrogen ions, wherein the β-Ga2O3 gallium oxide film is doped with hydrogen ions.

[0029] The atomic radius of hydrogen is 0.053 nm, and the atomic radius of oxygen is 0.074 nm. The atomic radius of hydrogen is slightly smaller than that of oxygen. That is, when hydrogen is injected into β-Ga2O3: (1) a portion of H replaces O in the β-Ga2O3 lattice position and becomes a substitutional impurity; (2) another portion of H fills the oxygen vacancy (Vo) in the original β-Ga2O3 lattice position; (3) the remaining H becomes an interstitial impurity between the atoms of the β-Ga2O3 lattice. It is doped with 1-8mm. Whether it is a substitutional H impurity (Ho) or an interstitial H impurity (Hi), the impurity energy level introduced by the H impurity is located at a shallow position in the Ga2O3 forbidden band, and its distance from the bottom of the conduction band is less than the distance between the oxygen vacancy and the bottom of the conduction band. The donor ionization energy of the substitutional H impurity (Ho) and the interstitial H impurity (Hi) is relatively small. The core of the present invention is to occupy oxygen vacancies.

[0030] In the embodiment of the present invention, based on the variable temperature Hall test, the donor ionization energy of oxygen vacancies is 0.24 eV, and the donor ionization energy of H impurities is 0.187 eV. In addition to the advantages of small donor ionization energy and easier ionization to form free electrons, the hydrogen impurities that fill the oxygen vacancies also have the following effects: (1) The quality of the single crystal is improved due to the reduction of oxygen vacancy defects. The X-ray diffraction test (XRD) confirms that the half-width of the diffraction peak corresponding to the same crystal plane becomes smaller, and the crystal quality is better. (2) Due to the reduction of oxygen vacancy defects, the effect of defects on electrons is reduced, and the mobility of electrons is increased.

[0031] The third aspect of the present invention discloses an application of a β-Ga2O3 film injected with hydrogen ions, wherein the β-Ga2O3 film injected with hydrogen ions is used to prepare various devices, and the β-Ga2O3 film injected with hydrogen ions has a higher electron carrier concentration; a greater electron mobility; a smaller resistivity; and a reduced donor ionization energy, thereby generally improving the n-type electrical properties of the non-doped β-Ga2O3 film. Therefore, the quality of the body region of a high-voltage and high-power device or the body region of a photodiode prepared using a β-Ga2O3 film containing hydrogen ions will be better.

[0032] In order to accurately perform the Hall electrical test, the present invention selects the "lower titanium / upper aluminum" metal ohmic electrode structure. Metal titanium with a small metal work function (Ti's work function is 3.84 eV) is selected. The small metal work function is conducive to forming a good ohmic contact with the medium-doped n-type β-Ga2O3. Metal aluminum is deposited on top of the metal titanium. The aluminum (Al) work function is also small, but the core role of aluminum in this application is that the aluminum resistivity is small, that is, the overall resistivity of the "lower titanium / upper aluminum" ohmic contact structure is reduced. Table 2 Comparison of n-type electrical properties of β-Ga2O3 thin film before and after ion implantation at a test temperature of 300K

[0033] See also Figure 1 and Figure 2 , (1) Ion implantation of H + Ions exist in three forms: filling Figure 1 The oxygen vacancies generated by MOCVD at high temperature are characterized; they replace the O in β-Ga2O3 and become interstitial impurities in β-Ga2O3. (2) Ion implantation of H + The ions are distributed in the entire non-doped gallium oxide epitaxial layer in a longitudinal Gaussian pattern. (3) The metal electrode ohmic contact with the "titanium below and aluminum above" structure is adopted. Titanium with a work function of 4.33 eV forms an ohmic contact with n-type β-Ga2O3, and aluminum plays a role in further reducing the overall electrode resistivity.

[0034] Depend on Figure 3 It can be seen that the undoped β-Ga2O3 film obtained based on MOCVD is a single crystal with a {100} crystal plane, and its crystal quality is good. + The highest X-ray diffraction peak of the β-Ga2O3 film before ion implantation appears on the (400) crystal plane, and its full width at half maximum (FWHM) is 0.01844. + After ion implantation, the β-Ga2O3 film is still a single crystal with a {100} crystal plane, and the injected H effectively fills the oxygen vacancy defects caused by the original MOCVD high temperature, so the crystal quality is slightly improved. + Before ion implantation, the highest X-ray diffraction peak of the β-Ga2O3 film is still located at the (400) crystal plane, and its full width at half maximum (FWHM) decreases from 0.01844 to 0.0183. The half widths of the diffraction peaks of the (600) crystal plane and the (800) crystal plane also decrease slightly. After ion implantation, the overall crystal quality is slightly improved. Since the half width decreases less, the slight improvement in crystal quality can be attributed to the reduction of oxygen vacancy defects.

[0035] Figure 4 Yes H + The diffraction peak and half-height width of the X-ray diffraction (XRD) rocking curve θ pattern at the (400) crystal plane before and after ion implantation. It can be seen that after ion implantation, H + After ion implantation, the X-ray diffraction peak did not become higher, but it became narrower, so the overall full width at half maximum (FWHM) became narrower and the crystal quality was slightly improved.

[0036] Depend on Figure 5 It can be seen that (1) in the vacuum variable temperature Hall test temperature range of 70 K ~ 660 K, the electron concentration in the β-Ga2O3 film increases with the increase of the test temperature; (2) in the vacuum variable temperature Hall test temperature range of 70 K ~ 660 K, the H + The electron concentration before ion injection is much smaller than that of H + Electron concentration after ion implantation; (3) H + Before ion implantation, the electron concentration in the β-Ga2O3 film is: When the test temperature is 70 K, the electron concentration is: 9.2×10 13 cm -3 When the test temperature is 660 K, the electron concentration is: 5.5×10 16 cm -3 ; (4) H +After ion implantation, the electron concentration in the β-Ga2O3 film is 8.7×10 15 cm -3 When the test temperature is 660K, the electron concentration is: 5.2×10 17 cm -3 That is: H + Ion implantation effectively increases the electron concentration (by about 10 times).

[0037] Depend on Figure 6 It can be seen that: (1) In the vacuum variable temperature Hall test temperature range of 70 K ~ 660 K, the electron mobility in the β-Ga2O3 film decreases with the increase of the test temperature. This is because the high test temperature increases the probability of electron scattering and reduces the mobility; (2) In the vacuum variable temperature Hall test temperature range of 70 K ~ 660 K, H + The electron mobility before ion implantation is significantly lower than that of H + Electron mobility after ion implantation; (3) H + Before ion implantation, the electron mobility in the β-Ga2O3 film is: 110.7 cm 2 / Vs, when the test temperature is 660K, the electron mobility is: 9.7 cm 2 / Vs; (4) H + After ion implantation, the electron mobility in the β-Ga2O3 film is: 191.3 cm 2 / Vs, when the test temperature is 660 K, the electron mobility is: 18.2 cm 2 / Vs. That is: H + Ion implantation effectively increases electron mobility (by about 2 times).

[0038] Depend on Figure 7 It can be seen that: (1) In the test temperature range of 70 K ~ 660 K of the vacuum variable temperature Hall test, the resistivity of the β-Ga2O3 film decreases with the increase of the test temperature. This is because the high test temperature increases the electron concentration, so the resistivity decreases; (2) In the test temperature range of 70 K ~ 660 K of the vacuum variable temperature Hall test, the H + The resistivity after ion implantation is significantly lower than that of H + Resistivity before ion implantation; (3) H +Before ion implantation, the resistivity of the β-Ga2O3 film is: 1912.1 Ω*cm when the test temperature is 70 K, and 90.9 Ω*cm when the test temperature is 660 K; (4) H + After ion implantation, the resistivity of the β-Ga2O3 film is 310.9 Ω*cm when the test temperature is 70 K, and the electron mobility is 12.1 Ω*cm when the test temperature is 660 K. + Ion implantation effectively reduces the resistivity of the gallium oxide film (by about 7 times).

[0039] Figure 8 and Fig. 9 They are H + The natural logarithm of the Hall electron carrier concentration ln (n) before and after ion implantation changes with the test temperature (1000 divided by the test temperature in Kelvin units, 1000 / T), and the donor ionization energy is estimated based on the linear regression formula of ln (n) to 1000 / T. The smaller donor ionization energy indicates that the n-type β-Ga2O3 film is easier to activate its electrons, making the electrons become free carriers. Figure 8 and Fig. 9 It can be seen that: H + After ion implantation, the donor ionization energy decreased from 0.24 eV to 0.187 eV, and the donor ionization energy was effectively reduced by 22.1%.

[0040] Based on the above analysis, the present invention proposes an innovative method for preparing β-Ga2O3 thin film implanted with hydrogen ions, which aims to + ) injection technology is used to effectively fill the oxygen vacancy defects inside the non-doped β-Ga2O3 gallium oxide grown homoepitaxially based on MOCVD technology, thereby significantly improving the electron concentration and electron mobility of the material. During the injection process, hydrogen ions are injected into the β-Ga2O3 film under the action of high energy, forming two types of H impurities: interstitial H impurities (Hi) and substitutional H impurities (Ho). Interstitial H impurities refer to hydrogen ions injected into the interstitial positions of the lattice, while substitutional H impurities refer to hydrogen ions replacing the positions of O atoms on the β-Ga2O3 lattice. In this process, some substitutional H impurities (Ho) successfully compensated for the oxygen vacancy defects in gallium oxide, thereby reducing the adverse effects of defects on the electrical properties of the material.

[0041] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a β-Ga2O3 thin film implanted with hydrogen ions, characterized in that: The following steps are involved: Step 1, growing a non-doped β-Ga2O3 gallium oxide thin film on a Ga2O3 substrate; Step 2, pretreating the non-doped β-Ga2O3 gallium oxide film; Step 3, injecting monovalent hydrogen ions into the pretreated non-doped β-Ga2O3 gallium oxide film to form a β-Ga2O3 film injected with hydrogen ions, wherein the concentration of monovalent hydrogen ions in the β-Ga2O3 film injected with hydrogen ions is 1-8ppm.

2. The method for preparing a β-Ga2O3 thin film implanted with hydrogen ions according to claim 1, characterized in that: In step 3, the injection temperature is room temperature.

3. The method for preparing a β-Ga2O3 thin film implanted with hydrogen ions according to claim 1, characterized in that: In step 3, the implantation energy is 90 KeV.

4. The method for preparing a β-Ga2O3 thin film implanted with hydrogen ions according to claim 4, characterized in that: In step 1, the thickness of the epitaxial layer in the non-doped β-Ga2O3 gallium oxide film is 400 nm.

5. The method for preparing a β-Ga2O3 thin film implanted with hydrogen ions according to claim 4, characterized in that: In step 3, the implantation depth is 400 nm.

6. The method for preparing a β-Ga2O3 thin film implanted with hydrogen ions according to claim 1, characterized in that: In step 3, the injection angle is 0°.

7. The method for preparing a β-Ga2O3 thin film implanted with hydrogen ions according to claim 1, characterized in that: In step 1, the Ga2O3 substrate is grown by metal organic chemical vapor deposition to obtain a non-doped β-Ga2O3 gallium oxide thin film.

8. The method for preparing a β-Ga2O3 thin film implanted with hydrogen ions according to claim 1, characterized in that: In step 2, the pretreatment process is to use acetone, ethanol and deionized water for ultrasonic cleaning in sequence.

9. A β-Ga2O3 thin film implanted with hydrogen ions obtained by the preparation method according to any one of claims 1 to 8, characterized in that: The β-Ga2O3 gallium oxide film is doped with hydrogen ions.

10. An application of the β-Ga2O3 thin film implanted with hydrogen ions as claimed in claim 1, characterized in that: The β-Ga2O3 film implanted with hydrogen ions is used to prepare the body region of a high-voltage and high-power device, or the body region of a photodiode.

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